The present invention relates to telecommunications devices and, in particular, to an improved automatic power control loop for a wireless telecommunications transmitter.
The Federal Communications Commission (FCC) regulates the use of the radio frequency (RF) spectrum in the United States. Users of allocated handwidth of the RF spectrum must take measures to ensure that radiated emissions inside and outside the allocated bandwith are maintained within acceptable levels to avoid interfering with other users' operating in the same or other bandwidths. For example, users of cellular telephone systems must ensure that they are compliant with the level of radiated emissions allowable inside or outside the channels they have been assigned.
Cellular telephones such as, for example, CDMA (code division multiple access) or TDMA (time division multiple access ) cellular telephones, include power amplifiers in the transmitter in which the power amplifier can be driven beyond a point where acceptable out of channel emissions are maintained. This is primarily due to the increased distortion output levels of the power amplifier at high powers.
Thus, regulating the transmitted signal power can reduce the amount of interference and spectral regrowth to a desired level. Certain wireless telecommunications transmitters, such as those used for cellular telephony, employ a transmit power control loop to regulate the transmitted signal power. In the mobile radio standard IS-95, for example, poorly regulated transmit power at the wireless terminal can lead to near-far effect at the base station demodulator, thus degrading the performance of the system. Similarly, in IS-136, drift in the transmit power loop can cause additional interference in the uplink channel, as well as spectral regrowth.
Regulating the transmitted signal power requires measurement or estimation of the actual transmitted signal power. However, this can be difficult, because modulation schemes such as DQPSK introduce signal power variations on the order of several decibels. On the digital side, this is due largely to the use of a square root raised cosine filter for pulse shaping as is required by the IS-136 standard, for example. FIG. 1 illustrates an instantaneous transmit power fluctuates between −4 and −22 dB for eight times overshaping. FIG. 2 illustrates the corresponding signal constellation. Again, there is considerable fluctuation about the points of interest.
Because of this relatively wide fluctuation in the transmit power, sampling the power output one time is not sufficient for fast and accurate convergence of the APO loop. One solution to this problem has been to perform signal power averaging via an analog RC filter or in the digital domain, by digitizing the output power. This latter method requires fast analog-to-digital converters. Either method, however, has proven to be expensive in terms of performance, accuracy, and board space. Consequently, many manufactures have settled on sampling the power only once, thus avoiding the averaging process. However, this can lead to inaccuracy in estimating the transmitted signal power and slowing the APC loop's convergence time considerably.
As such, there is a need for an improved transmit power control loop. There is additionally a need for an improved system and method for estimating transmit signal power.